Actuator units for injecting fuel are very frequently used as solenoid actuators, e.g. in automotive engineering. Examples are, for instance, the actuation of fuel injection valves or control of the adjustment of cam pieces for inlet and outlet valves of engines.
As illustrated schematically in FIG. 1, a solenoid actuator comprises an electrically conductive excitation winding 10, a ferromagnetic circuit 20 having a moving armature 30, and a ferromagnetic return. In the unactivated state of the actuator, there is an air gap 21 between the armature 30 and the ferromagnetic return, and said gap can amount to between 50 μm and about 1 mm, depending on application. By way of example, the armature 30 is connected to a nozzle needle 31 or the like. In general, the armature 30 is held in an idle position by means of a return spring 15, in which case the air gap 21 is open.
When a voltage is applied to the excitation winding 10, a magnetic fields builds up as the current flow increases in the ferromagnetic circuit 20. This field gives rise to a force which tends to reduce the air gap 21. As long as the magnetic force is smaller than the opposing force of the return spring, the armature 30 does not move out of its idle position. If the magnetic force exceeds the spring force as the excitation current rises, the armature 30 moves toward the ferromagnetic return of the ferromagnetic circuit 20 until, finally, the air gap 21 has reached its minimum, with the armature 30 running up against a stop (not shown in FIG. 1), for example.
The time for this movement determines the switching time of the actuator unit. It depends decisively on the rate at which the magnetic force can be built up. One problem is that the excitation current builds up more slowly at a lower voltage and that, as a result, the armature moves more slowly. In the extreme case, this can lead to the armature 30 no longer reaching the stop or no longer leaving the idle position, e.g. in the case of an undervoltage, as can occur in the onboard electrical network of a motor vehicle.
In order to prevent this situation, the selected inductance of the magnetic circuit, i.e. of the excitation winding, can be as small as possible, for example. However, this leads to an increase in the current through the excitation winding. As an alternative or in addition, the supply voltage can be increased, leading to a more rapid rise in current for a given inductance of the excitation winding. Where the actuator unit is used in a motor vehicle, the onboard electrical voltage is normally 12 V. An increase in voltage would therefore require an expensive voltage transformer.